Conventionally, a semiconductor device having a vertical semiconductor element used for a power circuit such as an inverter or a DC-DC converter of an EHV has been known. In this semiconductor device, an IGBT is used as a vertical semiconductor element. In order to reduce a loss in a semiconductor device having an IGBT, an element structure called a field stop (hereinafter referred to as the FS) has been proposed and put to practical use. Specifically, a back surface of a substrate where an element is formed is shaved, and then an annealing process is performed after ion implantation of impurities such as phosphorus (P) or selenium (Se) into the back surface is performed. Thus, a FS layer having an impurity concentration greater than a raw material concentration of the substrate is formed. Such a FS layer reduces an extension of an electric field, prevents a breakdown voltage from being degraded in spite of the thinning, and reduces a loss.
However, when the FS layer is formed by ion implantation of phosphorus, it is formed at a shallow depth and sensitive to damage on the back side because the implantation depth is shallow. Therefore, if the damage extends to a depth greater than the FS layer, collector leakage occurs, and fabrication yield decreases. In contrast, when the FS layer is formed by ion implantation of selenium, the implantation depth is deep, and fabrication yield is improved. However, since selenium is not used in a normal IC manufacturing process, special equipment for handling selenium is required.
In this situation, the patent document 1 has proposed a technique for forming a deep FS layer by performing an annealing process after performing a doping process with proton using an accelerator. Further, the patent document 2 has proposed a technique for forming a FS layer by ion implantation of phosphorus in addition to proton. Furthermore, the patent documents 3 and 4 have proposed a technique for forming a FS layer with a multilayer structure by performing implantation of proton multiple times.
However, when proton is used as proposed in the patent documents 1-4, the activation ratio significantly decreases with an increase in the dose amount. Therefore, a long irradiation time is required to obtain practical concentration, and productivity is reduced.
FIG. 15 is a diagram showing the peak concentration and the activation ratio with respect to the dose amount of proton when a doping with proton is performed at an accelerating voltage of 4.3 MeV. As shown in this diagram, the activation ratio of proton significantly decreases with an increase in the dose amount of proton. In particular, at a high impurity concentration of about 1×1015 cm−3 necessary to form a FS layer, since the activation ratio is low, a proton irradiation time becomes long. For this reason, if a FS layer is formed by simply using proton, the manufacturing cost increases due to low productivity.
In the method disclosed in the patent document 2, a FS layer is formed by using phosphorus in addition to proton. However, the dose amount of proton is constant regardless of whether phosphorus is implanted or not, and the doping is performed so that the proton concentration peaks at a deep position. Therefore, the problem of the long proton irradiation time is not solved, and an electric field concentration occurs at a boundary between a FS layer and a drift layer, so that a breakdown voltage is reduced. Further, a surge is likely to occur at the time of the switching. Therefore, it is desired to reduce a switching surge without an increase in the manufacturing cost.
The above explanation is made by taking an IGBT as an example of a vertical semiconductor element. However, the same problems as discussed above can occur in a freewheeling diode (FWD), DMOS, and the like where a FS layer can be formed.